Biomedical imaging techniques such as X-ray imaging, MRI, fluorescence and optical coherence tomography, NIR absorption imaging, etc., providing global morphological/density/absorption changes of the hidden components are based on retrieving information from the photons that have travelled inside the sample of interest. These techniques, in general, provide information on morphology or density, either based on the response of labels (e.g. dyes, fluorophores), or changes in the bulk properties of the materials but with no specific information on the chemical composition of the samples.
Photons in a multiple scattering media contain both Rayleigh and Rarnan scattered photons, recording Rarnan scattered light yields the structure and chemical nature of the molecules. In general, Raman spectroscopic observations are made at fixed collection angles, such as, 90°, 135°, 180° and transmission techniques. The problem with the geometry specific collection is that, it restricts the observations of Raman signals either from or near the surface of the materials. However, Raman signals of objects are generally scattered in all 360° angles or planes and therefore these signals, in principle, can be retrieved irrespective of the illumination or collection geometry. Signals can be obtained from all the observable angles from all the sides.
Since the multiply scattered light contains both Rayleigh and Raman scattered photons, recording Raman scattered light has been explored for identifying the structure and chemical nature of the molecules. Examples of known techniques that record Raman scattering include but are not limited to spatially offset Raman spectroscopy (SORS) and transmission Raman spectroscopy (TRS). SORS works on the principle of backscattering collection geometry wherein the scattering from the surface along with the Raman signal of the sample located deeper in the sample, contribute to the scattering. U.S. Pat. No. 7,911,604, assigned to The Science and Technology Facilities Council, discloses a method and an apparatus for screening objects using Raman scattering methods to detect the presence of predefined substances or classes of substances. The predefined substances may be hazardous, toxic, or explosive. Radiation is supplied to an incident region of an object. Scattered light is collected from a collection region on the surface of the object spaced from the incident region. The characteristics of the scattered light include Raman features related to the surface and predefined substances. The Raman features allow the presence, or not, of the predefined substances to be determined. One of the primary disadvantages of SORS is that the detection is restricted to a specific experimental geometry, which involves strict description of finite distance between light input and collection of scattered light. For example, SORS works only in the backscattering geometry with fixed orientation of incident and collection being either in the same axis or plane with varying distance between incident and collection locations.
Transmission Raman spectroscopy (TRS) is another technique adopted for screening objects. Raman signals are obtained from the transmission side of the sample. TRS cannot distinguish the individual layers of different chemicals in a multi-component layered system. U.S. Pat. No. 8,054,463, assigned to The Regents of the University of Michigan, discloses a method and a system for measuring sub-surface composition of a sample. An illumination area of a sample is irradiated using a light source and the light scattered from a plurality of emitting surface areas of the sample is received. Each emitting surface area of the sample is at a different location. For each emitting surface area, spectral content information associated with received light corresponding to that emitting surface area is determined, and composition information corresponding to a sub-surface region of the sample is determined based on the determined spectral content information. The Regents' Patent is restricted to backscattering and transmission geometry. The illumination and collection geometries are located in the same plane. The maximum offset allowed, between the illumination and collection geometry is 90°. Further, the collection geometry is restricted to only 2D plane. Hence, the image obtained is predominantly a 2D image which is then reconstructed to obtain a 3D image. The depth of detection is only up to 22 mm.
The current 3D imaging techniques are based upon observing the changes of the optical properties of tissues/materials based on absorption values or density differences. However, chemical specific information cannot be retrieved using such methods. Raman spectroscopy can readily yield molecular specific information. However, in order to obtain a complete shape of an object concealed within another layer, it is pertinent to acquire signals globally, i.e. from all the sides, planes and angles. Hence, there is a need for a method that can not only identify concealed samples but also profile them based on their shapes at various levels of depth.